How to join two vse video edit projects?
I'm assuming you have File1 and File2, and want to transfer the clips from File1 to File2.


Open File2 and select File > Append ShiftF1.
Navigate to File1 and "open" it. In Append mode we can view the contents of the file. Click on the scene folder of the file and select the scene, which you want to append. You can see the name of the scene in File1, if you open it and look in the top Header/Info Bar.
After selecting and appending the extra scene you can switch to it in the current File2. Since both scenes in File1 and File2 had the same name, the appended scene from File1 was renamed to Scene.001.

Open the scene originally from File1 (but now in File2), select all strips and copy them with Ctrl C.
Open the other scene and paste the strips.